1. Field of the Invention
The present invention relates to a low-noise optical fiber amplifier for use in a wavelength division multiplexing (WDM) optical transmission apparatus that uses a single-mode optical fiber as a transmission line.
2. Description of the Related Art
The use of an optical fiber amplifier in WDM optical transmission apparatuses triggers researches into an erbium-doped fiber amplifier (EDFA). Also, an increase in communication results in a need for more channels in a WDM optical transmission apparatus, which draws a lot of interests in a wide-band optical fiber amplifier. Further, much research is made on long-distance transmission of a wide-band optical signal. For the long-distance transmission of a wide-band signal, a method of using a low-noise optical amplifier, a method of inducing a Raman gain in an optical line, or a method of setting a distance between repeaters to be about 50 km is used.
In the case of a terrestrial system, a distance between optical repeaters must be more than 80 km, the use of a single-mode optical fiber (SMF) is considered, and a network must be reconstructed whenever a channel is added or dropped.